In several schools I know, the uniforms changed as the student's got older. In the school my sisters go to, when you get to year ten you get a different jumper and polo shirt (heaven knows why), and it's the same with a school several of my friends went to. When they got to year ten they wore a-line kilts instead of the normal ones because they were more flattering and grown up. This could be the case with the Hogwarts robes, maybe they have closed ones in the lower years, say from first to fifth, and then open ones in sixth and seventh? That could explain why Ron's shirt was untucked and Snape wasn't wearing trousers. But it could also be nonsense :P

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This is extremely complicated. XD I think Jo wasn't very consistent in her image, which is what's causing all of the confusion.